Ambient outside air is drawn into an air compressor in the process of compressing air. All of the constituent gases and vapors in the ambient atmosphere, including moisture (water vapor), enter the compressor in the same proportion as the proportions in the outside ambient.
For example, if a compressed air system provides air at a normal gauge pressure of 120.0 psi, and the ambient atmosphere has an absolute pressure value of 14.7 psi, the absolute pressure has increased by a factor of about nine. Vapors in the atmosphere, notably water vapor, which may be at a partial pressure below the dew point of the temperature of the ambient atmosphere, are compressed to a partial pressure which is increased nine-fold, and may greatly exceed the dew point for air at the temperatures in the compressed air system. Hence, the vapors, notably water vapor, condense in the compressed air system, causing liquid water to collect at low points in the system.
Liquid water has a density that is greater than air, and hence acts very differently from air when the water flows through valves, conduits and orifices in a compressed air system. Indeed, if a mixture of compressed water and air flow through an orifice or valve, the air may drive discrete masses of water at high speed, which can erode internal surfaces causing noise and damaging systems.
In addition, water corrodes various materials of systems using compressed air, and most dangerously can freeze and reduce or negate brake action in air brake systems.
Hence, it is necessary to rid compressed air systems of liquid water. Compressed air systems on trucks, for example, generally have manual water release valves which are opened occasionally or regularly to eject liquid water, or a mixture of water and air. Presently, trucks employed in the U.S. and elsewhere commonly use a desiccant dryer for removing moisture in compressed air received from the truck's compressor that provides pressurized air for the truck's brake system. Water removed from the compressed air by the desiccant dryer is periodically expelled from a sump of the dryer and thus from the system. The compressor operates under a load cycle controlled by a governor that senses the level of air pressure in a reservoir. When pressure in the reservoir falls to a preset level, the governor calls for a supply of pressurized air from the compressor; when air pressure is restored, the governor orders cessation of the supply of pressurized air from the compressor. The compressor itself is often gear-driven by the internal combustion engine of the truck.